MATLAB: Summing with respect to values within variable instead of rows, columns, or equal windows

Question

I have a variable with a similar structure to the following but with many more rows:

a=
1  0.3
1  0.1
2  0.7
2  0.6
2  0.3
2  0.9
3  0.5
4  0.8
4  0.1
5  0.1
5  0.9
5  0.4

Say I want sums of column 2 with respect to each scalar (eg. 1, 2…5) so that I would get a new variable like this:

aa=
1  0.4
2  2.5
3  0.5
4  0.9
5  1.4

I have not found how to sum anything but rows and columns, so how should I go about summing based on values within a variable?

Answer 1

a=[
1  0.3
1  0.1
2  0.7
2  0.6
2  0.3
2  0.9
3  0.5
4  0.8
4  0.1
5  0.1
5  0.9
5  0.4]
uv = unique(a(:,1),'stable');
sidx = accumarray(a(:,1),a(:,2));
[uv sidx]

accumarray() to the rescue!